I have both. I'm unable to easily find any of my posts on the forum older than 2 years, so I can no longer link you to the test shots I took with the 135-600mm The 135-600 f6.7 lens came with a dedicated close up filter, which drops the minimum focus distance to 1.3 meters. Pictures are missing for many of the posts here on the forum that showcase the lens. On the off chance you did not click on the last link in the review,
it shows what the lens will do if you nail focus.
Here is a link to my post showing the 1.4x & 300mm on the K-1, and a
actual pixel crop here.
Chromatic aberration is much less on the 300mm. It is also much easier to achieve good focus and control shake with the 300mm & 1.4x than the 135-600.
The 135-300 likes a heavy duty tripod. My custom lens plate helps. Not an easy setup to carry distances. The 1.4x degrades image quality of the 300mm slightly, but it will usually give you many more keepers than you will get with the 135-600.
I find that a Pentax IR remote is helpful for both. The 135-600 is capable of nice images, but it's a lot of work, and took me a lot of practice. It has a nice long throw for manual focus. The 300mm's autofocus is very nice. Manual focus is harder due to the shorter throw.
Unless you really have a nostalgia streak and love playing with vintage lenses, I think you will be much happier with your results using the 1.4x & 300mm combination.
Here is a picture of my 135-600 setup, followed by a test shot of a crocheted pillow using the dedicated close up filter.
SMC Pentax 135-600mm F6.7 _RGK2721 - F16 - Close focus lens - crop
---------- Post added 11-29-18 at 09:31 PM ----------
Originally posted by WPRESTO The 1.7X TC has a good reputation. I've seen a number of better than acceptable images posted on PF that were taken with it. However, I would still opt for the new 1.4X on an APS-C body or a K1 set to APS-C crop mode plus the DA 300 f4.
If memory serves, you need to get manual focus close with the lens, then the 1.7x does the fine tuning. The 1.4x allows a AF lens to do the focusing. Much faster focus for objects at different distances. I put up with the vignette on the K-1 rather than use crop mode, as I find it's quite helpful to have those extra pixels come crop time, or when my composition is a bit off!